LLDPE films with improved impact strength

ABSTRACT

LLDPE films with superior impact strength are obtained by blowing lightly crosslinked LLDPE resins under film extrusion conditions typically used for blowing HDPE film.

FIELD OF THE INVENTION

The invention relates to compositions comprising a linear low densitycopolymer of ethylene and an alpha-olefin containing 4 to 10 carbonatoms (LLDPE), and to films of improved properties formed from suchcompositions.

BACKGROUND OF THE INVENTION

Linear low density polyethylene is ethylene copolymerized with alphaolefins of 3 to 10 carbon atoms. Improvements in impact resistance offilms of linear low density polyethylene (hereinafter "LLDPE") aredesirable. Normal LLDPE resins lacks the melt strength required to blowfilms under high stalk extrusion conditions, often used to blowHMW-HDPE, which result in excellent film impact properties. One recentexception is a DMAC cocatalyzed LLDPE resins. A high molecular weightcomponent of the DMAC cocatalyzed LLDPE resins is accompanied by anincreased melt strength required to be blown into film under high stalkconditions used in HDPE film production.

SUMMARY OF THE INVENTION

In the present invention, the melt strength of LLDPE is increased bytreatment with low levels of peroxides. Low levels of crosslinking ofthe LLDPE results with low levels of peroxides. This allows the LLDPE tobe blown into film under high stalk conditions. The treated LLDPE can befabricated with improved processability into blown films having improvedimpact properties. The impact strengths of these LLDPE films issignificantly higher than that of films from uncrosslinked LLDPE filmsblown under non-stalk conditions.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates viscosity of peroxide crosslinked LLDPE and is a plotof complex viscosity, poises vs. frequency, rad/sec.

FIG. 2 is Elasticity of peroxide crosslinked LLDPE and is a plot ofstorage modulus G', dyn/cm.sq vs. frequency rad/sec.

DETAILED DESCRIPTION OF THE INVENTION

The linear low density ethylene copolymers (LLDPE) of this invention arelinear copolymers of ethylene and a minor amount, preferably about 2 to10 weight percent of an olefin, preferably a 1-olefin, containing 4 to10 carbon atoms and having a density of about 0.9 to 0.94, preferablyhaving a density of about 0.90 to about 0.93, a melting point of about110° to 130° C., and a melt index of about 0.05 to 10. The LLDPE willcontain more than 70% ethylene units in the backbone. The preferredolefin comonomers are 1-butene, 1-hexene and 1-octene. The LLDPE may beprepared by any of various methods known in the art, e.g., by relativelylow pressure methods as disclosed, for example, in U.S. Pat. No.4,076,698. As is known in the art, these polymers or copolymers areformed in the presence of Ziegler-Natta, Metallocene or Phillipscatalysts containing titanium, chromium, zirconium, magnesium, andadmixtures thereof which give rise to the catalyst residues in theas-polymerized as-synthesized! resin.

The amount of peroxide used to treat the LLDPE to increase the meltstrength in this invention can range from 10 to 1000 ppm. However,preferably, the peroxide amount ranges from 20 to 500 based on the LLDPEweight. Most preferably, the peroxide of the blend is about 50-200 ppm.

The types of peroxides which are used are high temperature peroxidesthat can undergo almost complete decomposition at normal compoundingtemperatures (190°-260° C.). The half life temperature at 0.1 hoursshould be greater than 130° C. Half life temperature is the temperatureat which one half of the peroxide has decomposed. Suitable butnon-limiting examples of such peroxide are: dicumyl peroxide,2,5-dimethyl-2,5-di-(tert butyl peroxy) hexane, tert-butyl cumylperoxide, di-(2-tert-butylperoxy-isopropyl) benzene, di-tert-butylperoxide, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexyne-3, cumenehydroperoxide these contain 2 to 20 carbon atoms. The peroxide treatedLLDPE may be blended using any of various methods known in the art.

As a result of the treatment the melt strength of the LLDPE isincreased. Melt strength is determined as the complex viscosity measuredat 190° C. at a shear rate (frequency) of 0.1 l/sec.

In the peroxide treated LLDPE, the numerical values of this viscocity isin the range of 100,000 to 2,000,000 poises, preferably 200,000 to700,000 poises.

The peroxide treated LLDPE can be then blown into film under thefollowing conditions, which have been used to treat HMW- HDPE but notLLDPE.

Film Production

In accordance with the process of film production, films of 0.3 to 10.0mils can be produced; in some commercial applications films of about 0.5mils are required.

In accordance with the invention, a melt of the linear low densitypolyethylene is fed through a gap in an annular die for extrusion in theform of a tube, which is moved vertically upward. Pressurized air is fedto the interior of the bubble formed by the tube, which blows it to agreatly increased diameter and correspondingly reduced wall thicknessand results in biaxial orientation of the film. Cooling air is suppliedto the exterior surface of a bubble, while the extruded tube of moltenmaterial is being drawn. Further handling usually involves collapsingthe tube between a pair of roles to a flattened double-wall web at astage in the cooling at which the wall surfaces will not adhere to oneanother. The flattened tube is wound onto a roll and/or furtherprocessed.

Cooling air can be supplied to the exterior surface of a bubble by oneor more cooling rings, each of which discharges one or more annularstreams of cooling air for heat exchange engagement with the bubbleexterior surface. Often a primary ring in the immediate neighborhood ofthe die orifice is employed with a more powerful secondary ring spacealong the path of the bubble at a location at which sufficiently towithstand the force of the more powerful secondary ring air stream orstreams.

These air rings can be configured, prearranged, not only to cool, butalso to shape, the tube of molten resin. Controlling the configurationof the tube and bubble, by such air rings is described in U.S. Pat. No.4,118,453 which is incorporated by reference herein. The internalpressure of the tube is maintained by employing pressurized gas (air)during passage of the tube through the air rings. The apparatus in whichsuch means are used are sometimes referred to as "stalk extruders";stalk extruders are commercially available from Alpine.

Thus in accordance with the invention the process comprises extrudingthe peroxide treated LLDPE having complex ASTM D-4440/84! viscosity at190° C. and 0.1 radians l/sec greater than 1.5×10⁵ poises through anannular die to form an extruded tube of molten material, cooling theextruded tube while drawing the tube so cooled, expanding the tube toattenuate the walls thereof by introducing a gas in contact with theouter surface of said tube from a plurality of annular zones about saidextruded tube spaced along the axis thereof and being of increasingdiameter in the direction away from the point of extrusion; theplurality of annular zones can be provided by circular pairs of annularzones about said extruded tube. In U.S. Pat. No. 4,118,453, incorporatedby reference herein, as noted above, additional separate pairs ofcooling gas confined streams are directed against said film on each sideof a shape restricting surface which extend beyond the dischargeboundaries of the discharged confined streams; the said additionalcooling gas streams are passed in contact with the outer surface of saidfilm tube at each of said shape restricting surfaces to produce apositive gas pressure zone between said surface and said film materialand then said cooling gas is withdrawn from such contact between eachpair of adjacent cooling gas inlets.

In accordance with the invention the molten linear low densitypolyethylene, described above, is formed into a tube or bubble having atleast two different diameters, the smaller of the two diameters beingsubstantially that of the die and the second diameter of the bubbleexceeding that diameter of the die, with a frost height line downstreamof the portion of the bubble having said smaller diameter and downstreamof the portion of the bubble having said second diameter. The Frost lineis the line where the extruded tube or bubble changes from molten tosolid character.

While the diameter of the tube is that of the die, the stresses, as wellas machine direction (MD) orientation, in the melt relax; this stage ofthe process has been found to be critical to increase the MD tearresistance and impact resistance. As the tube diameter increases, thepressure increases within the bubble; that is the pressure differentialbetween the inside of the tube and the external surface of the tubeincreases as the diameter increases. The increase in diameter can be 3:1to 5:1 and up to 7:1 to 9:1 times the die diameter. This expansion inbubble diameter occurs before the melt turns into a solid. As suggestedabove, the frost line height is where the film is below its meltingpoint with no more expansion in the transverse direction and so noincrease in bubble diameter. The resulting films have thicknessesranging from 0.2 to 10.0, preferably ranging from 0.3 to 1.5, and mostpreferably ranging from 0.3 to 1.0.

The resulting films have improved impact resistance (ASTM D1709).

The impact strength of the high stalk extruded peroxide treated LLDPEfilm is at least 25 percent higher than the corresponding film preparedin a non-stalk manner without the benefit of peroxide treatment.

In addition to the LLDPE polymer component, the blend may also containany of various additives conventionally added to polymer compositionsfor different purposes, e.g. stabilizers such as antioxidants, pigments,etc. Improvements in impact properties of linear low densitypolyethylene are realized by blending linear low density polyethylenewith polymers, such as impact polystyrene or impactpoly(para-methylstyrene). Improvements in impact resistance of linearlow density polyethylene has been described to result from blendinglinear low density polyethylene with impact polystyrene or impactpoly(para-methylstyrene). Although the addition of impact polymers canimprove impact properties of LLDPE film, the impact polymers, onaddition to LLDPE, can reduce the stiffness (modulus) of the resultingfilm. In the literature, the use of microtalc has been described to actas an antiblocking agent in LLDPE production.

Printing of the resulting films can be undertaken by any conventionalmethod. Gravure printing is generally used for films. Gravure inkconsists of pigment (usually organic pigment), a resin binder, and avolatile solvent; it is quite fluid and dries entirely by evaporation.For high speed printing, the solvents are quite volatile and the inkingsystem must be enclosed. Cf. KIRK-OTHMER ENCYCLOPEDIA OF CHEMICALTECHNOLOGY, VOL. 19. page 155 Third Edition 1976!

The results of the invention can be realized with peroxides as the soleagents to increase melt strengths; accordingly, the LLDPE blend with thesmall amounts of peroxide are to be construed to exclude of other whenthe language "consisting essentially of" or "consisting of" is employed.

The following examples further illustrate the invention.

EXAMPLES ILLUSTRATING THE ESSENCE OF INVENTION

A granular LLDPE base resin (0.922 density, 0.7 MI, 25 MFR) wascompounded on a single screw extruder (2 inch Sterling) at 220° C. undernitrogen blanket with 100 ppm of peroxide (Trigonox 101) and 500 ppmeach of Irganox 1010 and Irgafos 168. The resultant pellets had a FI of11.3, MI of 0.26 and a MFR of 43.

The pellets were blown into 1 mil film on the Alpine extruder under thefollowing conditions: melt temperature=463° C., screw speed 136 RPM,output 94 lbs/hr, BUR 4:1. The film properties were as follows:

    ______________________________________                                               Frostline Height                                                                          Dart Impact                                                                              MD Tear                                                                              TD Tear                                  Sample Inches      F50 gms    gms/mil                                                                              gms/mil                                  ______________________________________                                        1      19          800        142    360                                      2      28          630        141    412                                      ______________________________________                                    

A comparative film sample produced with a non-stalk extrusion (2:1 BUR,1 mils, 120 lbs/hr) on a Brampton 2.5 inch extruder with the base resin(Mobil's NTX-097) would have a dart impact which is substantially poorer(dart impact of 178 gms). The impact strength is increased by over 250percent over the non-peroxide treated non-stalk film.

The attached graphs illustrate the substantial change in the rheology ofthe LLDPE resin after crosslinking. The low shear viscosity is increased3-fold while the low shear elasticity (storage modulus) is increasedover 15-fold. The LLDPE resin also becomes more shear thinning asevidenced by the higher slope in the viscosity shear rate curve. Bychoosing an appropriate base LLDPE resins for crosslinking one couldproduce easy processing LLDPE resins using this concept.

Thus it is apparent that there has been provided, in accordance with theinvention, a blend, that fully satisfies the objects, aims, andadvantages set forth above. While the invention has been described inconjunction with specific embodiments thereof, it is evident that manyalternatives, modifications, and variations will be apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended to embrace all such alternatives, modifications, andvariations as fall within the spirit and broad scope of the appendedclaims.

What is claimed is:
 1. A film produced by high stalk extrusion of aperoxide treated LLDPE resin wherein the peroxide treated LLDPE isextruded through a die and a bubble is formed with a diameter which isat least 3:1 times the die diameter, and a film is produced whichexhibits impact resistance that is at least 25 percent higher than acomparable LLDPE film (same thickness and density) made from untreatedLLDPE under non-stalk extrusion, 0.3 to 10 mils thick, comprising alinear copolymer of ethylene and an alpha olefin of 4 to 10 carbonatoms, contacted with 50 to 500 ppm of high temperature peroxidecompound(s), compound comprises 2 to 20 carbon atoms and exhibits a halflife at 0.1 hours which is greater than 130° C., wherein the copolymerexhibits a melt strength as determined by complex viscosity at 190° C.and 0.1 sec⁻¹ in the range of 100,000 to 2,000,000 poises and a meltindex of 0.05 to 10 (MI).
 2. The film of claim 1, wherein the copolymercomprises a monomer selected from the group consisting of 1-butene,1-hexene and 1-octene.
 3. The film of claim 1, with high temperatureperoxide levels between 50-200 ppm and viscosity at 0.1 sec⁻¹ between200,000 and 700,000 poises.
 4. A film product produced by a processwhich comprises providing a resin which is a linear low densitypolyethylene comprising at least about 80 percent by weight of ethyleneunits;treating the linear low density polyethylene with high temperatureperoxide such that a viscosity at 0.1 sec⁻¹ (190° C.) of100,000-2,000,000 poises is achieved, wherein the linear low densitypolyethylene exhibits MI of from 0.05 to 10; extruding said resinthrough an annular die to form an extruded tube of molten material toprovide the tube with a tube diameter which is substantially the annulardie diameter; while continuously extruding the tube, expanding the tube,downstream of said annular die, to attenuate the walls thereof to formthe tube of molten material into a bubble of a bubble diameter whichexceeds (1) the annular die diameter and (2) the tube diameter, andwherein the bubble diameter is at least 3:1 times the annular diediameter; wherein said bubble has a frost line which comprises ademarcation line between said molten material and solid biaxiallyoriented film, and producing said film of a thickness ranging from 0.3to 10.0 mils, exhibiting an improvement in impact strength, determinedby ASTM D-1709 of at least 25 percent over a comparable film made from anon-peroxide treated LLDPE under non-stalk film flowing conditions;wherein said bubble has a frost line which comprises a demarcation linebetween said molten material and solid film; and producing said film. 5.The film of claim 1, wherein the copolymer comprises 1-hexene.
 6. Thefilm product of claim 4, wherein the copolymer comprises 1-hexene. 7.The film product of claim 6, wherein the high temperature peroxide isused in an amount ranging from 10 to 1000 ppm.
 8. The film product ofclaim 6, wherein the high temperature peroxide is used in an amountranging from 50 to 500 ppm.
 9. The film product of claim 6, wherein thehigh temperature peroxide is used in an amount ranging from 50 to 200ppm.
 10. The film of claim 1 wherein the blow up ratio ranges from 3:1to 5:1 and up to 7:1 to 9:1.
 11. The film of claim 1 wherein the blow upratio ranges from 3:1 to 9:1.
 12. The film of claim 4 wherein the blowup ratio ranges from 3:1 to 5:1.
 13. The film of claim 1 wherein theblow up ratio ranges from 3:1 to 5:1 and up to 7:1 to 9:1.
 14. The filmof claim 1 wherein the blow up ratio ranges from 3:1 to 9:1.
 15. Thefilm of claim 4 wherein the blow up ratio ranges from 3:1 to 5:1.